Cleaning and polishing agent for dental prophylaxis



United States Patent 3,330,732 CLEANING AND POLISHING AGENT FOR DENTAL PROPHYLAXIS Joseph C. Viuhler, Indianapolis, Ind., assiguor to Indiana University Foundation, Indiana Memorial Union, Bloomington, Ind.

No Drawing. Filed June 11, 1964, Ser. No. 374,257. The portion of the term of the patent subsequent to June 21, 1983, has been disclaimed 7 Claims. (Cl. 167-93) This invention relates to a new agent for both cleaning and polishing teeth by dental prophylaxis and more specifically to a new form of zirconium silicate having a particular particle size distribution and surface configuration as the said cleaning and polishing agent.

In a copending application, Prophylactic Paste Compositions, Ser. No. 314,602, filed Oct. 8, 1963, now patent No. 3,257,282, zirconium silicate is disclosed as a compatible carrier in combination with an anticariogenic agent (such as stannous fluoride or stannous zirconium hexafiuoride) for use in an anticariogenic prophylactic paste formulation. The present invention is concerned with the use of a particular type of zirconium silicate per se (i.e., having a defined particle size distribution and surface configuration) in an aqueous medium as an agent for both cleaning and polishing teeth without incurring any undue abrasion of the oral hard tissues (enamel, dentin, cementum).

The present invention is not concerned with therapeutic antican'ogenic effectiveness as such (although of course a cleaned and polished tooth structure is less susceptible to caries). Accordingly, the compatibility of the zirconium silicate (that is, the inverse of its ability to suppress ionization of an anticariogenic agent such as a fluoride salt) is not involved, and it has been found possible to select a particular type of zirconium silicate for maximized cleaning and polishing characteristics, even though that type of zirconium silicate might not exhibit maximized compatibility characteristics if an anticariogenic agent were to be utilized.

Dental plaque, food particles, exogenous stains,and other tooth surface pigmentation can be removed, to varying degrees, from teeth by means of a suitable dentifrice and toothbrush, as in ordinary daily brushings. Some enamel stains and pigmentation, however, are much too resistant to the abrasives found in conventional dentifrice formulations. Asa result, dentists must perform a prophylaxis in order to remove not only dental calculus (tartar) but the accumulated stains not satisfactorily removed by the daily use of a dentifrice. Most frequently, when either a dentifrice or the common abrasives used to perform a prophylaxis are employed, the teeth may be cleaned, but they exhibit a low luster or polish. For good dental esthetics, clean teeth with a high luster are desired, and the use of a particular distribution of specific particle size zirconium silicate as the cleaning and polishing agent in a prophylactic treatment as described herein accomplished these results in a manner superior to any agent heretofore evaluated.

The problems encountered in the relatively infrequent use of an abrasive in a prophylactic paste, for example a prophylaxis performed once or twice a year, are much different than the problems enoountered in the use of an abrasive as a constituent of a dentifrice for use two or three times a day, even though the desired end result of cleaning and polishing of enamel may be the same. Obviously, it is possible to use a more highly abrasive 3,330,732 Patented July 11, 1967 agent in a prophylactic paste than in a daily brushing with a conventional dentifrice. However, care must be taken in both cases to avoid excessive hard tooth structure loss at the expense of cleaning action. In general, it has been assumed that in attempting to remove the more difiicult forms of stain (e.g., tobacco, green, stannous fluoride, silver nitrate, etc.) there must be a major compromise between cleaning and tooth structure loss, or, in other words, that, in order to have maximum removal of these more difiicult stains and/ or pigmentations, tooth structure must be sacrificed. It has been found that the use of a particular particle size distribution of zirconium silicate with a specific surface configuration in a prophylaxis as described herein effectively removes even the most difficult enamel stains and pigmentation while minimizing hard tooth structure loss due to abrasion.

Another factor which should be considered in the development of a suitable prophylaxis is the ability to polish the teeth to a high luster, that is, to achieve smooth and highly lustrous enamel surfaces. Highly polished surfaces apparently are less receptive to retention of plaque and oral debris, and this factor is one of the motivating reasons for developing an abrasive which not only cleans effectively but which produces an exceptionally good luster. However, up to this time good luster could not be achieved without sacrificing cleaning or increasing abrasiveness.

Little, if any, relationship appears to exist either be tween the cleaning and the polishing abilities of dental abrasives or between the abrasiveness and the ability of the abrasive to produce a smooth tooth surface. For example, levigated alumina is an excellent polishing agent which produces a very smooth and shiny surface, but it is so abrasive that resultant tooth structure loss precludes its recommendation for use. Precipitated chalk, on the other hand, cleans fairly well but produces a rough tooth surface. Dicalcium phosphate is relatively unabrasive, but it fails to effectively polish. A mixture of insoluble sodium metaphosphate and tricalcium phosphate is a fairly effective polishing agent, but the combination is not a particularly good cleaning agent. The most common agent used in current day dental prophylaxis is flour of pumice. This agent is a fair cleaner, but it is highly abrasive and produces an extremely poor polish. In fact, recent studies have shown that the resulting tooth surface is so rough after being treated with flour of pumice that the occurrence of dental calculus, stains, pellicle, and other oral debris is significantly increased.

Thus, it would be highly desirable to provide an effective cleaning and polishing agent which minimizes tooth tissue damage. As has been indicated, a variety of compounds are known which clean well but which do not polish, or which polish well but do not clean satisfactorily. Prior to the instant invention, no formulation was known which would clean and polish to the desired extent with out simultaneously causing an undesirable degree of tooth structure damage. In accordance with the subject invention, it has been found that zirconium silicate, having a specific particle size distribution in the range of up to about microns mean diameter particle size, exhibits optimal cleaning and polishing characteristics for a prophylactic abrasive and that the particle size distribution is critically defined by a combination of various defined levels which are specific to particular cleaning and polishing functions. Moreover, it has been found that the surface configuration of the zirconium silicate particles has an important bearing on the combined cleaning and polishing abilities. Zirconium silicate particles prepared by attrition (e.g., hammer milling) have jagged surface configurations which act to predominantly clean until such time as the surface Wears smooth, at which time the smooth particles act to predominantly polish, whereas similar particles prepared by ball milling have generally smooth surface configurations which fail to effectively clean.

Accordingly, it is a primary object of this invention to provide a specific particle size distribution of zirconium silicate as a cleaning and polishing agent for use in prophylaxis.

It is a related object of this invention to provide zirconium silicate particles having a specific type of surface configuration as a cleaning and polishing agent for use in dental prophylaxis.

It is another related object of this invention to provide an agent for a dental prophylaxis which effectively removes the most difificult enamel stains and pigmentation while minimizing tooth structure loss.

It is a further object of this invention to provide an agent for a dental prophylaxis which polishes all accessible tooth surfaces to a high luster while effectively cleaning such surfaces.

It is still a further object of this invention to provide cleaned tooth surfaces which are polished and thereby less receptive to the retention of dental calculus, plaque, and other oral debris such as pellicle.

These and other objects, advantages, and features of the subject invention will hereinafter appear, and, for purposes of illustration, but not of limitation, exemplary embodiments of the subject invention are hereinafter described in detail.

Zirconium silicate is of course a well-known industrial abrasive which is used for the grinding and polishing of glass and ceramics; however, prior to the instant invention, this material had not been proposed for use as a dental cleaning and polishing agent in the oral cavity. Extreme hardness and abrasiveness characteristics exhibited by zirconium silicate (e.g., a hardness number of 7.5 on the Mohs scale for commercially available zirconium silicate, such as types used for grinding of glass) would suggest to one skilled in the art that zirconium silicate would seriously damage (i.e., abrade and scratch) tooth structure and would thus be unsuitable for use on the teeth.

In fact, experimentation has revealed that, unless zirconium silicate is presented to the oral cavity with a specific surface configuration and particle size distribution, it will be totally unsuitable for use as a dental abrasive. It has been found in accordance with the subject invention that the utilization of zirconium silicate prepared by attrition (to present an initial jagged surface configuration) and having specific defined levels of particles in the range of up to about 70 microns mean diameter particle size affords the desired prophylaxis characteristics, without undue abrasion of the teeth.

In order to assess the abrasiveness of a particular particle size zirconium silicate, an abrasion value can be calculated for tooth structure based upon the millimeters of tooth structure loss after treatment with a given abrasive. This method involves the polishing of a tooth, the thickness of which has been accurately determined when the tooth is mounted in a suitable container, such as a metal cup. The mounted tooth is polished with a selected abrasive by means of a conventional prophylactic cup. After a predetermined length of contact time (standard for all measurements), the thickness of the tooth is recalculated, and any loss of tooth structure is measured.

Typical abrasion values can thus be determined for various compounds which might be used as dental abrasives as a function of particle size. While abrasion values for all tested compounds tend to increase with increasing particle size, it is nonetheless possible to classify dental abrasives in terms of relative abrasion values over a predetermined useful range of particles (e.g., up to about microns mean diameter particle size). Thus, dental abrasives range from compounds having relatively low abrasion values (e.g., such substances as calcite) to those having relatively high abrasion values (e.g., such substances as flour of pumice and levigated alumina). Other corresponding compounds having relatively low abrasion values are calcium phosphate dibasic dihydrate, calcium phosphate tribasic, insoluble sodium metaphosphate, calcium pyrophosphate, etc., and likewise other corresponding compounds having relatively high abrasion values are calcium phosphate dibasic anhydrous, magnesium silicate, aluminum silicate, etc.

All of these indicated substances are either inert, poor, or only fair polishers when compared to zirconium silicate, with the exception of levigated alumina. Levigated alumina is a good polisher, but it is so abrasive that routine use of it, even infrequently in prophylaxis, is prohibitive. The uniqueness of the particular particle size distribution of zirconium silicate as described herein lies in its ability to produce relatively little abrasiveness while acting as a highly effective and specific cleaning and polishing agent.

The size of particles in an abrasive can be expressed in a number of different ways, one of the most common of which is mean diameter, i.e., the diameter above which one-half of the particles are larger and below which one-half are smaller. As hereinafter utilized, the term particle size refers to a mean diameter value.

The particle size of a particular abrasive is a highly reliable standard in predicting its abrasiveness. In the previously indicated abrasion value studies, abrasives were tested at particle sizes which varied by no more than plus or minus 3% in a 1 micron to 100 micron range. It was thus determined that flour of pumice and levigated alumina are relatively high in abrasiveness over the indicated range. Although levigated alumina is capable of producing a lustrous smooth surface, its abrasiveness precludes its use in dental prophylaxis. Moreover, flour of pumice, although a good cleaning agent, produces a dull tooth surface and fails to effectively polish. Calcite is similarly highly abrasive and will cut the dentin rapidly; moreover, it produces a dull tooth surface with considerable scratching. Apparently, the abrasiveness and scratchy-enamel characteristics of many abrasives (such as calcite) are due to impurities, one of the most common of which is free silica. For example, it is known that calcium carbonate with 5% silicia contamination is three times more abrasive than pure calcium carbonate. An important characteristic of zirconium silicate is its very low inherent concentration of free silicia (less than 0.04% thus apparently accounting in part for its ability to clean Without scratching dental enamel.

It has been found that zirconium silicate has a relatively low abrasiveness irrespective of its particle size over a range up to about 70 microns, although there is a gradual slight increase in abrasiveness toward the maximum useful range. It has also been found that the cleaning ability of jagged edge zirconium silicate (i.e., produced by attrition, such as hammer milling) increases with increasing particle size, while the polishing ability decreases with increasing particle size. The indicated inverse slopes for cleaning and polishing attributes of zirconium silicate as a function of its particle size suggest an optimum particle size range in which both cleaning and polishing characteristics are maximized. In accordance with the subject invention, this range has been found to be up to about 70 microns particle size, with highly specific distributions with this range being required. With particle sizes in excess of about 70 microns, the abrasiveness of the zirconium silicate is undersirably high and the polishing ability correspondingly decreases.

It has been found, in accordance with the subject invention, that certain combinations of relatively coarse and of relatively fine zirconium silicate may serve to enhance the polishing attributes without unduly hampering the cleaning ability of the combination, and vice versa.

More specifically, an intimate admixture of three levels of particle size distributions has been found necessary for effective: polishing and pellicle removal (Level I); cleaning and stain removal of enamel, dentin, and cementum dental hard tissues (Level H); and cleaning of nonorganic restorative materials, such as metal fillings, amalgams, plastics, silicates, etc., and of severe stains, such as green stain and stannous fluoride pigmentation (Level III).

The three levels are defined as follows:

Level: Particle size (microns) I Up to about 20. H From about 20 up to about 50. III From about 50 up to about 70.

In the preferred practice of the subject invention, the said levels are combined in a percentage ratio range of about 50% Level 1; 525% Level III; and balance Level II. For optimum effectiveness, a ratio of about 41:12:47 for Levels I, III, and II respectively should be utilized, as indicated in the preferred zirconium silicate formulation shown in Table I.

In accordance with the invention, the presence of particle sizes in excess of 70 microns should be restricted to not more than 2% and similarly the presence of particle sizes less than 10 microns should be restricted to not more than 25%, so that the major proportion of the particles will fall within a 10-70 micron particle size range.

The preparation of suitable particle size zirconium silicate can be accomplished by conventional techniques well known to the art. Basically, these techniques involve milling of zirconium silicate ore, followed by standard screen sieving to segregate the desired particle size (e.g., a standard 400 mesh screen retains approximately 95% of 35:3% micron particle size). While various milling techniques can be utilized, an attrition technique such as hammer milling is preferred since it yields the most desirable particle surface configurations for the purposes of the instant invention, although of course the invention contemplates other techniques which would yield the same ultimate configurations. The zirconium silicate particle resulting from hammer milling has a jagged edge surface configuration, and the surface protrusions are gradually eroded in the cleaning process so as to yield a relatively smooth surface configuration, for the ensuing polishing process. In contrast, ball milled zirconium silicate particles tend to exhibit relatively smooth surface configurations which are suitable for polishing but fail to effectively clean a wide variety of stains and pigmentation in prophylaxis. V

' Usually, the zirconium silicate is applied to the oral cavity as an aqueous slurry as a part of a conventional prophylaxis, although in some instances (such as the treatment of silver amalgams and of gold inlays) the zirconium silicate is introduced directly to the areas of treatment. In general, a minimum amount of water (e.g., parts water per 100 parts zirconium silicate powder) should be utilized for maximum cleaning, and more fluid mixtures (e.g., 25 parts water per 190 parts powder) should be utilized for maximum polishing. Of course, each treatment should be handled on an individual basis, depending on the exigencies of the circumstances.

It should also be understood that various materials (such as flavoring agents) could be added to the specifically defined zirconium silicate cleaning and polishing agent described herein. However, the invention contemplates the utilization of the said zirconium silicate as the essential cleaning and polishing medium as a part of a prophylaxis treatment.

Further details concerning the practice of the subject invention are revealed by the following description of exemplary clinical investigations.

EXAMPLE I In order to evaluate the ability of a specific particle size distribution of zirconium silicate to clean and polish teeth, it was compared with commercially available flour of pumice and lava pumice (which are utilized as conventional prophylaxis abrasives) in removing six different common forms of tooth stains or pigmentation. The tested flour of pumice, lava pumice, and zirconium silicate exhibited the percentage particle size distributions indicated in Table I.

In the comparative study, patients with stain or enamel pigmentation had either their respective left anterior maxillary central incisor, right anterior maxillary central incisor, or their two mandibular central incisors cleaned with the three respective abrasives. The patients were randomly divided in order to evaluate the cleaning and polishing abilities of the three abrasives by randomly selecting the different morphological types of teeth for evaluation by the diflerent abrasives. All of the abrasives were formulated for application in aqueous medium, and all were applied as a pasty slurry (i-e., at the same minimum powder to water ratio). The teeth of the patients were polished exactly for thirty seconds by conventional prophylaxis and were then evaluated for effectiveness of cleaning.

Table 11 records the results of this comparative study. Out of 544 subjects, a total of 61 patients had teeth with green stain. This type of stain, along with black stain and stannous fluoride pigmentation, are known to be the most diflicult to remove from enamel surfaces. The zirconium silicate abrasive cleaned 60 out of the 61 cases, while the lava pumice and the flour of pumice cleaned only 34 and 19 cases, respectively. Similarly, with respect to black stain (which most frequently results from an excessive use of tobacco), the zirconium silicate abrasive was effective in 12 out of 14 cases, while the lava pumice and the flour of pumice were effective in only 5 and 2 cases, respectively. Similar proportionate results showing the superior cleaning ability of the zirconium silicate abrasive are reported for removal of pellicle, yellow stain, brown stain, and stannous fluoride pigmentation.

EXAMPLE II A particularly severe case of green stain occurred in the anterior teeth of a 19 year old male who stated that he had never been to a dentist and in addition that he had only brushed his teeth once or twice a year throughout his life. The severity of the green stain in this patients mouth was so extensive that consideration was being given to capping the anterior maxillary incisors. For comparative purposes, the left central incisor of this patients mouth was treated for ten seconds with the particular particle size distribution zirconium silicate abrasive shown in Table I.

A minimum amount of water (15 parts water per parts abrasive) was utilized. The right central incisor was similarly treated with the flour of pumice distribution shown in Table I. With the indicated treatments, no visually observable green stain remained on the left central incisor, while there was no visually observable evidence of removal of green stain from the right central incisor.

EXAMPLE IH Pigmentation, apparently caused by utilization of topically applied stannous fluoride, was observed in a patient whose four maxillary anterior incisors were badly pigmented in the cervical areas. Attempts to remove this pigmentation with conventional abrasives and with bleaching agents met with only moderate success, as the majority of the pigmentation remained, notwithstanding a lessening in the intensity of its color. Prophylaxis treatment with the particular particle size distribution of zirconium silicate (shown in Table I) removed for all practical purposes all stannous fluoride pigmentation from the enamel surfaces of this patient. The zirconium silicate abrasive also removed the pigmentation from the light White area of the hypoplastic cervical enamel.

EXAMPLE IV A series of patients were clinically tested by prophylaxis with different abrasives in order to determine the relationship of the prophylaxis abrasive and the reformation of pellicle on the cleaned and polished tooth surfaces of the patients. Fifty patients between the ages of 22 to 24 years were divided into five equal groups on the basis of their oral hygiene and degree of pellicle at the initial examination. None of the subjects had anterior caries or anterior restorations. The maxillary and mandibular central and lateral incisors were given a thorough prophylaxis, with one of the five different abrasives listed in Table III. The first four of these abrasives are commercially available materials commonly utilized as dental abrasives. All of the patients were given new toothbrushes and were asked to refrain from using any dentifrice or dental floss during the study period. Instructions were given to each patient as to a suggested method of toothbrushing so as to help reduce their apprehension concerning mouth odor since a dentifrice was not to be used. The Table III data show marked differences between the five different abrasives with respect to their ability to reduce reformation of pellicle on a cleaned and polished tooth surface. Flour of pumice, a mixture of equal amounts of insoluble sodium metaphosphate and tricalcium phosphate (IMP/TCP), and tin oxide all produced enamel surfaces in which pellicle reformed within about one week. Levigated alumina produced an enamel surface which delayed the reformation of pellicle for about seventeen days, but the particular particle size distribution of zirconium silicate (shown in Table I) was by far the most effective in that reformation of pellicle was delayed for about one month.

EXAMPLE V A related indicator of the polishing effectiveness of zirconium silicate was observed in the treatment of a patient who complained of enormous accumulations of material on his teeth. Upon examination and questioning, it was found that the patient had been receiving a commercial oral antibiotic for the past Week, and the accumulation to which he referred appeared to be the result of yeast overgrowth within the oral cavity. The subject had practiced excellent oral hygiene previously and had been under clinical observation for many years. The left and right mandibular teeth of this patient were treated with zirconium silicate and flour of pumice, respectively, by conventional prophylaxis, using the particle size distribution shown in Table I. The patient was then observed to determine the rate of reformation of materia alba on the lingual surfaces of the mandibular anterior teeth. The time required when materia alba could first be identified following the use of a flour of pumice abrasive was about 2.5 hours, but in the anterior teeth treated with the zirconium silicate abrasive the first observance of materia alba did not occur until after the passage of about 86 hours.

From the foregoing, it is apparent that the invention described herein provides a unique prophylaxis treatment predicated upon the utilization of a particular particle size distribution of zirconium silicate having a specific surface configuration as a cleaning and polishing agent. It should be understood that various changes, modifications, and alterations may be effected in the details of formulation and application for the zirconium silicate cleaning and polishing agent described herein, without departing from the spirit and the scope of the instant invention, as defined in the appended claims.

TABLE I Particle Size (Microns) Lava Flour of Zirconium Level Pumice Pumice Silicate Less More (percent) (percent) (percent) thantlian III 7O 2. 7 2.0 0.7 6O 1. l 6. 0 5. 7 60 50 9. 3 2. 7 5. 8 II 50 40 8. 1 8.2 10.0 40 37 3. 9 7.0 4. 3 37 35 6. 7 2. 4 3. 5 35 33 2. 6 4. 9 4. 2 33 3O 2. 4 6. 3 7. 1 3O 24 13. 6 10. 4 8.6 24 20 4. 1 10. 0 9. 5 I 20 1O 25. 0 20. 8 19. 3 10 20.5 19. 3 21. 3

TABLE II Number cleaned as function N0. with of abrasives No. of Type of stain, pel- Subjects Stain licle, or

pigmenta- Flour of Lava Zirconium tion Pumice Pumice Silicate 544 Green 61 19 34 60 Black 14 2 5 l2 Pcllicle 48 41 42 48 14 10 10 14 10 2 5 10 20 2 9 18 377 TABLE III Abrasive Flour of pumice What is claimed is:

1. An agent for both cleaning and polishing teeth comprising as its essential ingredient uncoated zirconium silicate, ZrSiO having a distribution of mean diameter particle sizes in a first level of up to about 20 microns, in a second level of from about 20 microns up to about 50 microns, and in a third level of from about 50 microns up to about 70 microns, the three levels being combined in a percentage ratio range of about 10-50% of the first level, 5-25 of the third level, and the balance of the second level.

2. An agent as claimed in claim 1 wherein the three levels are combined in a percentage ratio range of about 41:12:47 for the first, third, and second levels, respectively.

3. An agent as claimed in claim 1 wherein not more than 2% and not more than 25% of the zirconium silicate, ZrSiO particles are present as more than 70 microns and less than 10 microns mean diameter particle size, respectively.

4. An agent as claimed in claim 1 wherein the zirconium silicate, ZrSiO particles have a jagged edge surface configuration.

5. An agent for both cleaning and polishing teeth comprising as its essential ingredient uncoated zirconium silicate, ZrSiO particles having jagged edge surface configurations and having a distribution of means diameter particle sizes in a first level of up to about 20 microns, with not more than 25 0f the total particles being less than 10 microns, in a second level of from about 20 microns to about 50 microns, and in a third level of 9 from about 50 microns to about 70 microns, with not more than 2% of the total particles being more than 70 microns, the three levels being combined in a percentage ratio range of about 10-5015-25zbalance for the first, third, and second levels, respectively.

6. An agent as claimed in claim 5 wherein the said percentage ratio range is about 41:12:47.

7. A process for both cleaning and polishing teeth comprising the application thereto of an agent comprising as its essential ingredient uncoated zirconium silicate, ZrSiO particles having a distribution of mean diameter particle sizes in a first level of up to about 20 microns, in a second level of from about 20 microns up to about 50 microns, and in a third level of from about 50 microns up to about 70 microns, the three levels being combined in a percentage ratio range of about 10-50% of 10 the first level, 5-25 of the third level, and the balance of the second level.

References Cited UNITED STATES PATENTS 2,427,799 9/ 1947 Maloney 51-308 2,941,926 6/1960 Salzmann et a1. 16793 3,105,013 9/1963 Saul et a1. 167-93 3,151,027 9/1964 Cooley et al. 16793 FOREIGN PATENTS 180,531 1/1936 Switzerland.

LEWIS GOTTS, Primary Examiner.

ELBERT ROBERTS, Examiner.

R. L. HUFF, Assistant Examiner. 

1. AN AGENT FOR BOTH CLEANING AND POLISHING TEETH COMPRISING AS ITS ESSENTIAL INGREDIENT UNCOATED ZIRCONIUM SILICATE, ZRSIO4, HAVING A DISTRIBUTION OF MEAN DIAMETER PARTICLE SIZES IN A FIRST LEVEL OF UP TO ABOUT 20 MICRONS, IN A SECOND LEVEL OF FROM ABOUT 20 MICRONS UP TO ABOUT 50 MICRONS, AND IN A THIRD LEVEL OF FROM ABOUT 50 MICRONS UP TO ABOUT 70 MICRONS, THE THREE LEVELS BEING COMBINED IN A PERCENTAGE RATIO RANGE OF ABOUT 10-50% OF THE FIRST LEVEL, 5-25% OF THE THIRD LEVEL, AND THE BALANCE OF THE SECOND LEVEL. 